1. Field of the Invention
This invention relates to the testing of electronic digital apparatus. The invention particularly relates to the testing of such apparatus using a technique known as signature analysis.
2. Description of the Prior Art
In signature analysis bit sequences from an instrument under test are fed to an instrument known as a signature analyzer. Typically the signature analyzer incorporates a linear shift register which is clocked at the same rate as the bit stream under test. The linear shift register is provided with feedback from selected stages of the shift register and the feedback bits are summed in modulo 2 at the input to the shift register. A typical shift register will be 16 bits in length and at the end of a measurement sequence the 16 bits in the register comprise 4 groups in hexadecimal format which represent what is known as a signature. Each group of 4 bits represents an alpha numeric character and the four alpha numeric characters can be displayed on a suitable display device. The group of 4 alpha numeric characters are compared with the known good signature to thereby test the operation of the instrument. This method can be used to test a number of points within the circuit of an instrument. Typically the instrument manual will contain a list of signatures, one for each test point in the instrument, to enable easy checking of signatures expected at various locations. Signature analysis is a known technique which is now well documented in the art. Articles describing the technique can be found in Hewlett Packard Application Note 222-2.
Instruments which incorporate a processing capability can have self-test routines for use with a signature analyzer. Read only memory (ROM) associated with the processor is used to hold the software that runs on the processor. Signature analysis which involves data compression to reduce complex serial data stream patterns of a given length at a circuit node to a 4 digit decimal signature referred to above can be used to test such arrangements. The test is carried out by causing the contents of the ROM to appear at each data pin of the ROM. Then between each cycle of the full ROM's address space the pattern of data appearing at a pin is fed into a signature analyzer. The signature analyzer compresses the data as explained above to generate the hexadecimal signature referred to above. A problem with this technique, however, is that it suffers from the disadvantage that many signatures are dependent upon the contents of the ROM. As such they need updating and distributing each time an update of software or firmware occurs.